Power supply unit arrangements comprising two switching converters that alternatively provide a controlled output voltage are known. They serve, inter alia, for redundant current supply or supplying an electronic device having different power consumptions in different operating states.
FIG. 1 shows such a power supply unit arrangement 10 comprising a first switching converter 11 and a second switching converter 12. The first switching converter 11 provides a controlled output voltage, for example, a controlled DC voltage DC-out1 at a first output 13 of the power supply unit arrangement 10. The second switching converter 12 equivalently provides a second output voltage, for example, a controlled DC voltage DC-out2 at a second output 14. Both switching converters 11 and 12 are supplied with a common supply voltage, for example, a rectified intermediate voltage Prim-DC from an input 15.
In that example, the first switching converter 11 and the second switching converter 12 are each a so-called DC/DC converter which, on the basis of an amplitude of a control signal Prim-Control, sets the magnitude of the voltage output at the output 13 and 14, respectively. For the closed-loop control thereof, the power supply unit arrangement 10 in accordance with FIG. 1 comprises on the secondary side a first control loop 16 for the first switching converter 11 and also a second control loop 17 for the second switching converter 12. By the control loop 16, the voltage at the first output 13 is controlled to a predetermined value depending on a predefined reference voltage Vref_1a. Equivalently, the voltage at the second output 14 is controlled to a predetermined value by the second control loop 17 using a second reference voltage Vref_2a. 
Depending on the device supplied by the power supply unit arrangement 10, the output voltages DC-out1 and DC-out2 can either be of identical magnitude or of different magnitudes. By way of example, it is possible to provide a voltage of 12 V at both outputs 13 and 14. In the arrangement described, the second switching converter 12 furthermore provides a primary auxiliary voltage for operation of the two switching converters 11 and 12.
What is problematic in the power supply unit arrangement 10 described with reference to FIG. 1 is, inter alia, the fact that the switching converters 11 and 12 and also the associated control loops 16 and 17 generally have to be supplied with an operating voltage even if an electronic device connected to the outputs 13 or 14 does not take up any power from the respective switching converter 11 or 12. If the first switching converter 11 is, for example, a main converter that operates a computer in a normal operating state and the second switching converter 12 is an auxiliary converter that provides an operating voltage in an energy saving mode, the first switching converter 11 and the associated control loop 16 always consume a small amount of energy even in the energy saving mode of the computer.
A further problem is that one or both switching converters is or are often operated with a comparatively low output power that is significantly less than the rated output power thereof. The energy efficiency of switched-mode power supply units, comprising one or a plurality of switching converters, depends inter alia on the output power of the switching converters. This relationship is illustratively represented in FIG. 2.
FIG. 2 shows efficiency profiles I and II of the first and second switching converters 11 and 12, respectively, in accordance with FIG. 1. The power in watts [W] is plotted on the abscissa, while the efficiency in percent [%] is plotted on the ordinate. The efficiency profile I describes the efficiency profile of the first converter circuit 3, while the efficiency profile II represents the efficiency profile of the second converter circuit 4. It can be discerned, in particular, that the efficiency and, hence, the energy efficiency of switched-mode power supply units increases with increasing output loading. Precisely computers and other devices appertaining to information technology are often operated over a relatively long period of time in a low-load range far below the maximum output power of the switching converters used.
It is evident from FIG. 2 that the first switching converter 11 (profile I) appears to be expedient for a power consumption starting from, e.g., 35 watts, while the second switching converter 12 (profile II) appears to be more expedient for a power consumption of, e.g., 0 watts to 35 watts. That means that the two efficiency profiles I and II intersect at a point at an output power of approximately 35 watts. Consequently, operation of the second switching converter 12 (profile II) would be advantageous in a low-load range, while operation of the first switching converter 11 (profile I) would be advantageous in a normal or high-load range.
To improve the energy efficiency of power supply units, DE 10 2010 035 112 A1 discloses a closed-loop control circuit for an auxiliary power supply unit, comprising a control loop that controls a voltage converter of the auxiliary power supply unit to a setpoint voltage by a controlled variable. In that case, the control loop comprises an additional circuit designed to limit a change in the controlled variable upon provision of an external voltage of a different voltage source at a first output that outputs the voltage generated by the auxiliary power supply unit if the external voltage exceeds the setpoint voltage of the auxiliary power supply unit.
The known circuit enables outputting of a supply voltage of two different switching converters at a common output. In that way, an electrical power required on the consumer side can optionally be provided by an auxiliary power supply unit or a main power supply unit such that the respective power supply units can preferably be operated in a range with comparatively high energy efficiency. In the known power supply unit arrangement, the auxiliary power supply unit is held in an active state by an additional circuit to prevent a sudden voltage dip from the main power supply unit being switched off. What is disadvantageous about the known circuit is that, in the event of an abrupt rise in the load, the converter of the main power supply unit does not ramp up rapidly enough and the auxiliary power supply unit is thus overloaded and possibly switches off.
It could therefore be helpful to provide an improved power supply unit arrangement which enables a reliable and efficient supply of a consumer with a controlled output voltage in a low-load range. Preferably, the power supply unit arrangement should have a particularly high energy efficiency over a relatively wide output power range. In this case, the power supply unit arrangement should not rely on the provision of external control signals, for example, by the device supplied by it.